Electromechanical transducer



w. T. HARRIS 2,947,823

ELECTROMECHANICAL TRANSDUCER Filed Jan. 51, 1958 Aug. 2, 1960 INVENTOR www@ Z'HAM/,f

BY man M l ATTORNEYS ELECTROlWECCAL TRANSDUCER Wilbur T. Harris, Southbury, Conn., assignor to The Harris Transducer Corporation, Woodbury, Conn., a corporation of Connecticut Filed Jan. 31, 1958, Ser. No. 712,433

' 1 Claim. (Cl. 179-110) This invention relates to electromechanical transducers andmore particularly to such devices which respond to fluctuating mechanical forces essentially along a single axls.

'I t. is afgeneral object of the invention to provide an improved transducer for converting mechanical forces to an electrical output. Y

It is another general object of the invention to provide a-transducer responsive to mechanical forces which, while :being small and easily assembled, is extremely sensitive to small forces Vor to slight changes in applied forces.

It is aj-specific object of one aspect of the invention to provide a compact microphone.

' Itis another specific object of a second aspect of the invention to provide a compact and rugged accelerometer. VItis a further specific object of the invention to provide a compactrand sensitive stethoscope employing a transducer in accordance with another aspect of the invention.V

Still further specic objects lare to provide magnetostn'ctive rand;electrostrictive devices meeting thev above objects- Y l yOther objects and various further features of novelty `and invention will be pointed out or will occur to those skilled in the `art from la reading of the following specification in conjunction with the accompanying drawings. In said drawings, which show, for illustrative purposes only, preferred forms of the invention:

, Fig v1 isa perspective view, partially in cross-section, showing a-microphone in accordance with one embodiment ofthe invention; n

i Figs/ 2aY and Y2b Yare Venlarged sectional views of parts offgalternativeversions of the ring-like impedance ele- :the alternating-current. impedance bridge will vary with ment employed in the microphone of Fig. 1;

Fig. 3 is a top view of internal parts of the microphone of Fig. l;

Fig. 4 is a perspective View, partially in cross-section, showing an accelerometer in Iaccordance with a second embodiment of the invention;

Fig. 5 is a perspective View partially in cross-section of -a stethoscope in accordance with a further embodiment of the invention;

Fig. 6 is a top view of the segmented conical coupling member Iand ring-like impedance element for use in either the accelerometer of Fig. 4 or the stethoscope of Fig. 5.

Briefly, in accordance with the general aspect of the invention, a radially strictive transducer is provided for Aconverting varying axially directed mechanical forces to correspondingly varying changes in electrical quantities. 'Ihe transducer comprises a stiff thin-Walled substantially conical coupling member for receiving the mechanical forces, Iapplied `axially and at the apex of the cone. A ring member of radially strictive material is fixed to the conical coupling member, and an electrical output is developed in response to mechanical-stress changes in the material of the n'ng member. Any mechanical forces, such as those related to yacoustical pressures, inertial accelerations, or mechanical vibrations during the reception of mechanical energy, are transformed to circumferential strains in the ring member and a corresponding electrical output, observable at the leads to the ring member.

Referring to Figs. l `and 3, a microphone 10 is shown coupledto 'an electric circuit 12. The microphone, 10 comprises ya thin-walled hollow metallic cone 14 with a radially strictive .ring-shaped' impedance element 16 bonded to its base periphery. The mass of element 16 preferably .substantially exceeds that of cone 14. A cellular rubber pad 18 supports the impedance element 16 in ahousing 20. The wall of the housing 20 opposite the hollow metallic cone 14, is a perforated screen 22 to permit the access of acoustical energy to the hollow metallic cone. 14. ,Y

For the form of Fig. 2a, the impedance element 16 is magnetostrictive and comprises a ring-shaped core of magnetostrictive material, such yas -a spiral-laminated development of magnetostrictive strip material or, ias in the form. shown; al ferrite 16a having magnetostrictive properties, Toroidal winding 16b is disposed about the ferrite ring v16a'to form a substantially toroidal inductor. A plastic potting or encapsulation completely encases both the toroidal winding and :the ferrite ring, lending ruggedness tothe structure while not affecting its magnetic propergties. The magnetostrictive impedance element 16 is bonded :to the base periphery of the hollow metallic cone 14, as by way of, av peripheral shoe element 15, las shown in Fig. 2. l Y

Y During operation, acoustical pressures passing through the screen 22 exert iluctuating mechanical forces on the hollow metallic cone 14. These forces are predominantly axially of the cone 14 and are transformed by cone 14 intoiluctuating predominantly radial forces on the magnetostrictive impedance element 16, due to the relatively great inertia of ring 16 compared with that of cone 14.

` The radial components of these forces introduce circum ferential strains in the ferrite ring 16a, 'and a voltage output .is developed 4for amplification or yother processing at'ircuit l1 2- r V`Nqtonly. are applied iiuctu-ating pressures reected as a Vfluctuating Avoltage output, vbut the strain changes in the orew16a also eiect corresponding changes in the reluctance of the element 16. Thus, when the electrical circuit v12 is an lalternating-current bridge, with the impedance element 16 as one ,of the arms of the bridge, a modulatornis', attainable, that is, the output signal from the impedance of the magnetostrictive impedance element in response to the changes in `acoustical pressure, it being noted of course tha-t the carrier frequency is elevated above :the range of applied Iacoustic frequencies. Although the electrical circuit 12 is disclosed as an alternating-current impedance bridge, any suitable modulator which includes an alternating-current carrier source can equally be employed.

In the embodiment of Fig. 2b, the ring element 16 is piezoelectric and may comprise an 4annular core 17 of suitably polarized piezoelectric ceramic (such as barium titanate) having radially inner and outer foil electrodes 17'-17 to which the output lead connections are made. Other pants are as described previously and are therefore given the same reference numbers. Electrical capacitance changes and voltage fluctuations appear in the output of the piezoelectric version of Fig. 2b analogously to the impedance and voltage changes in the output of the magnetostrictive version, and conventional signal-processing techniques may be employed at 1 2.

Fig. 4 shows an accelerometer 30 connected to an electrical circuit 32 which may be a conventional alternating# current impedance bridge. 'Ihe accelerometer 30 comprises a radially split conical `spring 34 which defines a plurality of angularl-y spaced segments 36. Fixed to the Patented Aug. 2., 1960 base peripheries of each of the segments 36 is a mass 38. A lradially strictive impedance element 16', which may be similar to rthat Idescribed at 16 in Pigs. 2a and 2b, is bonded to the outer surface of each of the segments 36 (Eig. 6), the radius of ring 16 being such in relation to the size of segments 36 that it is bonded to each segment at a location intermediate mass 38 and the radially inner limit of the slits between segments 36. The split conical spring 34 is shown lixed to the housing 40 by means of a screw 42, a Washer 44 and a nut 46. It should be noted that the dimensions of the washer 44 and nut 46 are chosen to permit free flexure of the segments 36.

In operation, the accelerorneter 30 is sensitive to accelerations along the axes of the split conical springY 34 and of the impedance element 16. Whenever there is such an axial acceleration, :the masses 38 exert forces which are transformed to radial stresses in the core of the impedance element 16'. In the magnetostrictive version, the resulting circumferential strains change the magnetization of the core material, andthe impedance of the magnetostrictive impedance element 16 changes. This change in impedance disrupts the balance of the altermating-current impedance bridge of the electrical circuit 32. A measure of the imbalance indicates the degree of acceleration of the masses 38. Analogous outputs are developed for the piezoelectric modification.

Fig. shows a stethoscope 50 which is coupled to an electrical circuit 52, which may also be an alternatingcurrent impedance bridge. The stethoscope 50 is similar to the accelerator 30, Vand primed reference characters are employed to point upl the similarities.

In particular, a split conical spring 34 having segments 36', is bonded to a radially strictive impedance element 16" in the same manner `as the corresponding elements of the accelerometer 30 shown in Fig. 4. It should be noted, however, that there are 'no masses tixed to the base peripheries of the segments 36. These base peripheries 36 serve as fingers or feelers which sense vibrations in a contacted body. The split conical spring 34 is Xed to the housing S4 by means of the screws 58, the washer 60 and the nut 62. The base of the housing opposite the base peripheries of the segments 36' is covered with a pliable membrane 56 to permit a substantially free transmission of generally axially directed vibrations to the segments 36', and a sponge-rubber or similarly compressible ring 55 axially separates the rim 54' of the case 54 from the adjacent flattened ends 57 of the segments 36. Any vibrations are converted to circumferential strains in the core of the impedance element 16, and a correspondingly changing impedance is presented to the alternatingcurrent impedance bridge in the electrical circuit SZ.

In magnetostrictive versions of the invention, it should be noted that the core rings in each of the impedance elements may be magnetically biased for more reliable and sensitive operations. Magnetic bias is readily obtainable by using permanently magnetized ferrites, by employing an additional Winding connected to -a directcurrent source, or by superimposing a direct-current component on an alternating-current signal generated in the electrical circuits 12, 32 and 52.

It will be seen that I have described improved radially strictive transducersv Which convert mechanical forces to electrical impedances. 'I'he transducers shown are compact, rugged and highly sensitive, particularly to mechanical forces along the transducer axis. In all cases, the device transforms predominantly yaxial forces into radial deformations of a ring, and the mechanical advantage in such transformation is a function of the stiliness and apex angle of the cone. Preferably, the apex angle is large,` as shown, for greater sensitivity.

While the invention has been described in detail in connection with the preferred forms illustrated, it will be understood that modifications may be made Within the scope of the invention as defined in the claim which follows.

I claim:

A microphone comprising a conical surface member of a stiif material, a ring of magnetostrictive material having a cylindrical radially inner surface, a winding disposed about said ring for coupling to an electrical circuit, cylindrical shoe means concentric with said ring and located radially inside .the part of said Winding adjacent said radially inner surface, the periphery of said conical surface member abutting the inner surface of said shoe meansat a location intermediate the axial ends of said shoe means, bonding means securing said shoe means to said ring, and said ring having a mass substantially exceeding that of said conical surface member, whereby varying acoustical pressures exerted on said conical surface member may be transformed to a varying electrical output `of said Winding.

References Cited in the file of this patent UNITEDA STATES PATENTS y2,477,596 Gravley Aug. 2, 1949 2,487,962 Arndt Nov. 15, 1949 2,607,858 Mason Aug. 19, 1952 2,778,881 Fryldund Ian. 22, 1957 2,836,738 `Crownover May 27, 1958 

